Multistage electrodialysis procedures for desalination of aqueous salt containing solutions are well known. In accordance with these processes, two or more stages are provided in which electrodialysis cells are divided into alternate dilution and concentration chambers by alternating pairs of anionic and cationic membranes so that each chamber has two boundary members, one an anionic membrane and the other a cationic membrane. The former, upon application of an electric charge, will permit the passage of anions such as chloride or sulfate ions towards the anode. The latter will permit the passage of cations such as calcium or sodium ions towards the cathode.
This process, although very attractive, has not yet realized its full potential because of the tendency of insoluble salt formation in the concentration chamber. Usually these salts are formed from divalent ions present in the water to be treated. The concentration of these ions in the raw water is not sufficiently high so that the salt forms and precipitates. However, in the concentration chambers of multistage electrodialysis units, they become progressively more concentrated so that the salt ultimately precipitates.
It has therefore become the practice in this art to apply water softening procedures to the raw water so as to remove a large proportion of these divalent ions before subjecting the water to electrodialysis. This procedure, while effective, considerably increases the cost of desalination, so that electrodialysis suffers competitively when compared with other desalination processes, such as reverse osmosis or ion exchange.
Accordingly, much effort has been directed to improving electrodialysis procedures utilizing ion selective membranes in attempts to avoid the initial softening step.
Cationic and anionic membranes are well known in the art, and can be prepared by conventional procedures to have a variety of properties. Thus membranes can be prepared which will selectively retain or transport anions or cations. They can also be prepared to selectively retain or transport monovalent anions or cations. Membranes can be prepared which in an electrolytic cell will permit the passage of monovalent cations such as sodium at a greater rate than the passage of divalent cations such as calcium under the influence of an electric charge. Such membranes are known in the art as divalent permselective cationic ion exchange membranes.
Divalent permselective anionic ion exchange membranes which will permit the passage of monovalent anions at a faster rate than the passage of divalent anions are also known.
The divalent ion permselectivity coefficient, for example the coefficient for the calcium ion F.sub.Ca, or the sulfate ion F.sub.SO.sbsb.4 are defined as follows:
The divalent ion permselectivity coefficients referred to here in the specification and claims, i.e., divalent cation permselectivity coefficient (F.sub.Ca) and divalent anion permselectivity coefficient (F.sub.SO.sbsb.4) are defined as follows:
F.sub.ca : relative permselectivity of calcium ion to sodium ion through a cation exchange membrane calculated by the following equation under the measurement conditions set forth below: ##EQU1## wherein N.sub.Ca and N.sub.Na are normal concentrations of Ca.sup.+.sup.+ and Na.sup.+ ions, respectively, in concentration streams, and C.sub.Ca and C.sub.Na are those of Ca.sup.+.sup.+ and Na.sup.+ ions, respectively, in dilution streams; PA1 F.sub.so.sbsb.4 : relative permselectivity of sulfate ion to chlorine ion through an anion exchange membrane calculated by the following equation under the measurement conditions set forth below: ##EQU2## wherein N.sub.SO.sbsb.4 and N.sub.Cl are normal concentrations of SO.sub.4 .sup.-.sup.- and Cl.sup.- ions, respectively, in concentration streams, and C.sub.SO.sbsb.4 and C.sub.Cl are those of SO.sub.4 .sup.-.sup.- and Cl.sup.- ions, respectively, in dilution streams.
Measurement method:
Silver-silver chloride electrodes are set at terminal ends of an electrodialysis frame. The electrodialysis frame is divided into five chambers, namely the anode, dilution, concentration, dilution and cathode chambers, by placing ion-exchange membranes between the electrodes. There are placed in the order from anode of (a) an anion exchange membrane (Aciplex A-101; trade name of Asahi Kasei Kogyo Kabushiki Kaisha, Japan), (b) a cation exchange membrane of which relative permselectivity is to be measured, (c) an anion exchange membrane (Aciplex A-101) and (d) a cation exchange membrane (Aciplex K-101; trade name of Asahi Kasei Kogyo Kabushiki Kaisha, Japan), when F.sub.Ca is to be measured. In case of measurement of F.sub.SO.sbsb.4, the cation exchange membrane (b) is replaced by cation exchange membrane (Aciplex K-101) and the anion exchange membrane (c) by an anion exchange membrane of which relative permselectivity is to be measured. The ion exchange membranes each have an effective current flow area of 4 cm .times. 4 cm.
When F.sub.Ca is measured, an aqueous mixed salt solution (NaCl = 0.05N; CaCl.sub.2 .times. 0.05N) is passed through dilution chambers at the linear velocity of 10 cm/sec, and the concentration chamber is filled with the same aqueous mixed salt solution. For the measurement of F.sub.SO.sbsb.4, an aqueous mixed salt solution (NaCl = 0.05N; Na.sub.2 SO.sub.4 = 0.05N) is passed through dilution chambers at the linear velocity of 10 cm/sec, and the concentration chamber is filled with the same aqueous salt solution. Through anode and cathode chambers, respectively, are passed 0.1N aqueous sodium chloride solutions. Electrodialysis is carried out at a current density of 2.2 A/dm.sup.2 at 25.degree.C. Four hours after the start of current passage, the streams are analyzed for ion concentration and the values of F.sub.Ca and F.sub.SO.sbsb.4 determined.
The Invention
Conventionally in desalination by multistage electrodialysis, the cationic and anionic membranes utilized have been selected with no particular attention to their divalent ion permselectivity coefficients. It has now been discovered that desalination by multistage electrodialysis can be effected without an initial softening step, and without precipitation of insoluble salts in the concentration stream provided that the ion exchange membranes are selected with regard to their divalent ion permselectivy coefficients, and their position in the electrolytic cell.
One of the most important desalination procedures is the desalting of aqueous solutions, such as brine, containing gypsum. For convenience, the process of this invention will be principally described as it relates to this procedure.